1. Field of the Invention
This invention relates to the fractional extraction of butadiene from a mixture of hydrocarbons having primarily four carbon atoms per molecule. More particularly, this invention relates to the fractional extraction of butadiene in a hydrocarbon thermal cracking plant.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce individual olefin products such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In such olefin production plants, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil or natural gas liquids is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. This mixture, after preheating, is subjected to hydrocarbon thermal cracking at elevated temperatures of about 1,400 to 1,550° Fahrenheit (F.) in a pyrolysis furnace (steam cracker or cracker). Thermal cracking is not a catalytic process, as opposed to catalytic cracking.
The cracked product effluent from the pyrolysis furnace contains hot, gaseous hydrocarbons, both saturated and unsaturated, of great variety from 1 to 35 carbon atoms per molecule (C1 to C35). This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate product streams of high purity, e.g., molecular hydrogen, ethylene, and propylene. After separation of these individual streams, the remaining cracked product contains essentially hydrocarbons with four carbon atoms per molecule (C4's) and heavier. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a C5 and heavier stream is removed as a bottoms product.
The crude C4 stream has a variety of compounds such as n-butane, isobutane, 1-butene, 2-butenes (cis and trans), isobutylene, butadiene (1,2- and 1,3), vinyl acetylene, and ethyl acetylene, all of which are known to boil within a narrow range, see U.S. Pat. No. 3,436,438. Further, some of these compounds can form an azeotrope. Crude C4's are, therefore, known to be difficult to separate by simple distillation. This crude C4 stream is then typically processed for the recovery, among other things, of butadiene therefrom.
The incoming crude C4 stream, fresh from the cracking process, normally is subjected first to an operation designed to prepare that stream for the fractional extraction of butadiene there from. This preparation step for incoming fresh crude C4 feed is used for several purposes depending on the particular processing the stream has previously undergone. For example, this preparation step can include the removal of carbonyl compounds, the removal of vinyl acetylene and ethyl acetylene, the removal of remaining C5 compounds, and the like.
The thus prepared incoming crude C4 stream is then subjected to fractional extraction as a first step toward separating a 1,3-butadiene product from that crude C4 stream. This extraction step employs a solvent extraction process that produces a solvent extract stream that is elevated, “rich,” in its 1,3-butadiene content.
The dominating process for separating 1,3-butadiene from crude C4's is known technically as “fractional extraction,” but is more commonly referred to as “solvent extraction” or “extractive distillation.” However it is termed, this process employs an aprotic polar compound that has a high complexing affinity toward the more polarizable butadiene than other olefins in the crude C4 stream. Known solvents for this process include acetonitrile, dimethylformamide, furfural, N-methyl-2-pyrrolidone, acetone, dimethylacetamide, and the like. This process and the solvents used therein are known, see U.S. Pat. Nos. 2,993,841 and 4,134,795.
Simple thermal distillation, often called stripping, of the 1,3-butadiene rich solvent extract stream removed from the extractive distillation tower has been employed to form a 1,3-butadiene concentrate overhead stream and a separate bottoms solvent stream that is substantially reduced, “lean,” in its butadiene content. The lean solvent stream from this 1,3-butadiene concentration forming tower is recycled to the extractive distillation tower for re-use to extract additional 1,3-butadiene from incoming fresh crude C4 feed. The separate 1,3-butadiene concentrate stream from this concentration tower can be subjected to a water wash for the removal of traces of solvent.
Heretofore, the water washed 1,3-butadiene concentrate stream was introduced into a lower portion, the lower 25% of the vertical height, of an upstanding simple thermal distillation tower, often called a finishing tower, for the formation of the 1,3-butadiene overhead stream that is the desired product of the overall 1,3-butadiene extraction process (extractive distillation/concentration/finishing). The bottoms stream from this finishing distillation tower was typically recycled upstream of the crude C4 stream preparation step aforesaid, for example, by mixing with incoming fresh crude C4 feed and re-processing starting with the extractive distillation tower.
It has been surprisingly found that by carrying out the operation of the aforesaid finishing tower in a manner that produces a bottoms stream from that tower that is rich in cis-butene-2 and physically removing that bottoms stream from the overall 1,3-butadiene extraction process, additional fresh crude C4 feed can be introduced into the 1,3-butadiene extraction process at a volumetric rate that is significantly greater than the volumetric rate of the removed and separated finishing tower bottoms stream.
Thus, by this invention, the capacity of the extractive distillation tower for processing incoming fresh crude C4 feed is increased in an amount that is substantially greater than the amount of the finishing tower bottoms recycle stream that is redirected out of and away from the overall 1,3-butadiene extraction (production) process.
This invention, therefore, allows for the introduction into the foregoing 1,3-butadiene production process of an amount of butadiene containing fresh crude C4 feed that is substantially greater than the amount of the displaced finishing tower bottoms stream, thereby substantially increasing both 1) the amount of butadiene containing crude C4 material that can be fed into the 1,3-butadiene production process in general, and the extractive distillation tower in particular, and 2) the amount of 1,3-butadiene product that can be recovered from the overall 1,3-butadiene production process, all without physical modification of the extractive distillation tower, or other equipment used in the process, to increase its operating capacity.